The ?-herpesvirus herpes simplex virus 1 (HSV-1) is a medically important pathogen in healthy individuals and immunocompromised patients. Primary infection begins with inoculation of mucosal or skin epithelial cells; the virus is then transferred by peripheral nerve axons to neurons of sensory ganglia where latency is established. HSV-1 undergoes phases of productive lytic infection and genome silencing reflecting highly regulated interactions between the virus and differentiated host cells. However, knowledge of the molecular requirements for infection and viral restriction by host cells within their distinctive tissue microenvironments is limited. We will study HSV-1 pathogenesis in human tissue, using skin xenografts in the severe combined immunodeficiency (SCID) mouse model, as a novel approach to address this gap. Our primary objective is to assess functions of the latency-associated transcript (LAT) locus. The LAT locus has been investigated extensively in HSV-1 neuropathogenesis but little is known about the contributions of LAT locus elements to productive infection in peripheral tissues. Our preliminary data show defective replication in skin of both KOS and 17syn+ mutants that carry a 203bp ?Pst core LAT promoter deletion, compared to their rescue viruses. This unexpected observation suggests that requirements for HSV-1 pathogenesis are more stringent within the human skin microenvironment. Notably, LAT functions were necessary in differentiated human skin cells in vivo even though SCID mice lack adaptive immunity. These findings warrant investigating how LAT locus transcripts and microRNAs (miRNAs) encoded in this region influence infection of human skin cells. Our approach emphasizes the opportunity to use deep sequencing methods for detecting and quantifying viral and cell mRNA transcripts and miRNAs in infected tissues, and to follow the kinetics of their expression in relation to pathogenesis in human skin in vivo. Our specific aims are designed to characterize how LAT locus transcripts (Aim 1) and LAT and cellular miRNAs produced during lytic HSV-1 infection (Aim 2) contribute to skin pathogenesis in vivo. The progression of infection is monitored using quantitative assays for viral replication and lesion formation in skin xenografts over a 3-14 day interval. These aims combine strategies of open-ended transcriptome and miRNA profiling of infected skin to analyze the effects of LAT locus mutations with the evaluation of mutants that have targeted disruptions of specific LAT miRNAs. We predict that LATs and LAT miRNAs will both regulate infection and contribute to achieving the optimal balance with intrinsic antiviral defenses, so that lesions are formed and HSV-1 can be transmitted to new hosts. More broadly, this study in the SCID xenograft model has the potential to yield key insights about HSV-1 disease not identified previously because functions encoded in the LAT region, especially long non-coding RNAs and miRNAs, are likely to be specific for the human host. From a public health perspective, understanding HSV-1 skin infection and the role of LAT locus functions is relevant for designing novel antiviral drugs or attenuated vaccines.